The invention relates to sensor devices.
Seismic data are typically gathered during a survey using an array of detectors. In the case of marine surveys, hydrophones measure pressure fluctuations in the water caused by incoming seismic waves. Geophones measure vector quantities such as displacement, velocity or acceleration. In the case of marine surveying, a plurality of cables or streamers, which are spaced apart typically by about 100 metres, are towed behind a boat. Each cable has detectors spaced along the cable at intervals. In the case of land surveys, a geophone array is laid out on the ground with the geophones in an approximate grid formation. The detector array detects seismic signals from reverberations of a signal from a seismic source, such as an airgun for marine surveys. In Ocean Bottom (OBC or OBS) acquisition, a detector array is fixed on the sea bed. In this case, the source may be an airgun mounted on a boat.
The differences between the different types of surveying are considerable, mainly of the different propagation characteristics of different physical environments. Different problems exist, and have different effects to mitigate, in different surveying environments. For instance, marine surveys involve very significant reflections at the sea bed, where there is a large difference between the acoustic velocities either side of the boundary. Land surveys are subjected to unwanted propagation, including shear waves (also known as S-waves) and Love waves, which are not experienced in marine surveying. As such, different hardware and different data processing techniques are used in the different surveying environments.
There are four main wave types detected by land survey sensors. These are S-waves, Love waves, P-waves (also known as primary waves, because they are the first detected after an earthquake) and Rayleigh waves. Rayleigh and Love waves are horizontally-propagating surface waves, whereas S-waves and P-waves propagate through bodies and thus are of most interest to seismological surveyors. P-waves are compressional waves, Rayleigh waves have a complex motion involving compressional modes and S-waves and Love waves have no compressional component.
Rayleigh and Love waves are together termed ‘ground-roll’. These waves are generally considered to be noise and obscure to some degree reflections from boundaries in the geology being surveyed. As such, the removal of the effects of ground-roll from land surveying data is of considerable interest. Various techniques have been developed that reduce the amplitude of the ground-roll and thus enhance the reflections.
A commonly used technique involves deployment of dense field arrays or single sensors of vertical component geophones. Ground-roll is attenuated using dip or velocity filters in further data processing. In order to avoid spatial aliasing, this technique requires at least two geophones per lowest wavelength inline, i.e. in the source receiver direction. Side scattering of the seismic energy can be reduced using two-dimensional array of geophones. Ideally such an array has similar sampling intervals both inline and transverse to the seismic source.
When data from three-component geophones is available, polarisation filtering can be applied. Polarisation filtering identifies the Rayleigh wave part of the ground-roll from the 90 degree phase-shift between its horizontal and vertical component, thereby allowing it to be removed by data processing. Polarisation filtering has been discussed by Kragh and Peardon, 1995, “Ground roll and polarisation”, First Break, 13, 9, pages 369-378, First Break, 13, 9, 369-378. ISSN (printed): 0263-5046 and more recently has been promoted by Kappius and Crewe, 2002, “Adaptive Vector Filters for Ground Roll Reduction”, CSEG Geophysics, available at the time of writing at http://www.cseg.ca/conferences/2002/2002abstracts/Kappius_R_Adaptive_Vector_Filters_for_Ground_Roll_Reduction_.pdf. An omniphone is a device with horizontal and vertical geophones and a signal processing unit that applies a polarisation filter to the data before output, as is discussed by Lawton, D. C. and M. B. Bertam, 1991, available at the time of writing at http://www.crewes.org/Reports/1991/1991-01.pdf.
Since the slowly propagating ground-roll is locally attenuated, the minimum required spatial sampling depends on the fast seismic reflection. As such, a survey carried out in this way typically requires relatively few geophone stations. However, polarisation filtering techniques do not work well in areas with a complicated near surface as they do not take into account the scattered Love waves present on the horizontal component data.